1. Field of the Invention
This invention relates to a method and plant for preheating and calcining pulverulent or granular raw material, such as cement raw meal prior to passing it down to a kiln for further heat treatment.
2. Description of the Prior Art
Prior art plants for heat treating pulverulent or granular raw material such as cement raw meal have been known to consist of one or more preheater strings, a rotary kiln and a cooler for cooling the product treated in the kiln. The preheater strings are fed with either hot kiln gases exiting from the upper material inlet end portion of the kiln, or with spent cooling air which passes directly from the cooler to a specified string. Each string may further be provided with a calciner for completely or partially calcining the raw material fed to the string before the material passes to the kiln. Commonly assigned U.S. Pat. No. 3,864,075 filed Apr. 29, 1974, is an example of a kiln plant according to the prior art. The kiln plant consists of two multi-stage preheater strings, one of which is fed with kiln exit gases -- called the "kiln string" -- and the other -- called the "cooling air string" -- is provided with a calciner coupled between its last stage and its penultimate stage and is fed with spent air from the cooler. The raw material fed to each preheater string is at least partially preheated and passes to the calciner in the cooling air string for joint precalcining. From the calciner, the materials pass through the last preheater stage of the cooling air string to the upper material inlet end portion of the kiln.
Another prior art plant construction is disclosed in U.S. patent application Ser. No. 626,478, filed Oct. 28, 1975 now U.S. Pat. No. 4,045,162, dated Aug. 30, 1977. The plant is provided with calciners in each of the preheater strings and precalcining takes place separately in each individual string before the treated material passes to the kiln.
In both of the above-mentioned plants the desired percentage of calcining of the material treated in the calciners is not always achieved. Further, very hot kiln exit gases must be fed to the kiln string to attain an efficient preheating and/or calcining temperature. This creates a risk of heat damage to the material inlet end portion of the kiln and its surrounding structure. I have invented a method and a plant which avoids these drawbacks and provides an improved approach for preheating and precalcining raw material such as cement raw meal.